Cryogenic techniques and apparatus to separate methane and/or ethane (as well as other lighter gases such as carbon dioxide and nitrogen) from a gas containing a mixture of hydrocarbon gases have long been known. Such an apparatus is commonly called a "demethanizer". While the term "demethanizer" will be used throughout, it is to be understood that the apparatus and methods described may also be used in other applications whereby the demethanizer would be operated as a deethanizer. Typically, the purpose of a gas processing facility is to receive a gas from a transmission line, efficiently cool and depressurize the gas, extract the more valuable heavier components (ethane and heavier hydrocarbons, referred to as "ethane plus"), reheat and recompress the gas, and feed it back into the transmission line.
Typically, in such an operation, an inlet gas is introduced into a process facility at a high pressure. The gas is then allowed to expand and cool in various stages, and the liquid and gas fractions are then separated into different streams. After several stages of expansion and cooling, the various hydrocarbon streams are introduced into a fractionation column, or demethanizer, at different heights in the column. The methane gas is then separated from the heavier components of the gas, with the methane component exiting the fractionation column through the top of the column as a residue gas, and the heavier components exiting the column at its lower portion (collected as a liquid).
In the fractionation column, there is typically located a packing, which may be in the form of a number of contact trays. However, any suitable packing or construction can be used that promotes contact between the vapour and liquid flows. Thus, conventional Raschig rings or other standard packing materials can be used in the column. This packing is designed to increase mass transfer contact between the falling liquids and rising gases within the column, which increases the efficiency of liquid-gas separation in the fractionation column. As well, there is usually located at the upper portion of the fractionation column an enlarged empty "disengagement section". Typically, the stream entering the fractionation column at its upper portion (in the disengagement section) comprises between about 5% and about 50% liquid phase when it enters the fractionation column. The disengagement section allows the liquids entering the fractionation column space to separate or "deentrain" from the vapours with which it is mixed. The empty disengagement section is designed to alleviate potential problems of carryover of hydrocarbon liquids into the demethanizer overhead stream (which is supposed to be vapour). This results in a loss of hydrocarbon liquid product. Conventionally, the disengagement section is of a large diameter (about 10 to 18 feet) and is about ten to fifteen feet in height.
It has also been known that, to increase the efficiency of a standard demethanizer, to increase the amount of heavier hydrocarbons removed, additional packing or contact trays may be added in a separate column, which is connected with the fractionation column. In order to obtain a useful increase in efficiency, however, it has been thought that at least eight additional trays, and as many as twenty additional contact trays were required in this separate column. Therefore, the modification of existing facilities to obtain an increase in efficiency has previously been very expensive, since a whole new column (and all of the additional equipment associated with it) was required.
One known process for separating different components in a hydrocarbon gas stream is disclosed in U.S. Pat. No. 4,278,457 (to Campbell and assigned to The Ortloff Corporation). The claimed improvement in that patent is to divide the feed gas stream into two separate streams, one of which is cooled and then depressured through an expander, while the other of which is cooled to a greater degree and is then depressured through a simple expansion valve. The separate first and second streams are then supplied to the fractionation column at different feed points.
This U.S. patent discloses a number of different examples, which either operate at a flow rate of 6588 lb moles/hr or at a quarter of this flow rate, i.e. 1647 lb moles/hr. Interestingly, in three examples, where the input gas flow is separated into three separate inputs, and where a substantial portion, as much as 76%, of the input gas flow, passes either into the middle or bottom inlet of the column, the lower flow rate of 1647 lb moles/hr is given. Moreover, this patent does not discuss, in any way, details of the column design, in terms of diameter required at different heights, number of trays or number of trays between different inlets.
Even more particularly, in this Ortloff patent, there is no detailed direction as to the number of trays that might be required between a middle inlet and a top inlet to a fractionation column, where the major portion of the gas is supplied to the middle inlet, and a small portion is provided, substantially in the liquid phase, to the top inlet.
Supplying a substantial portion of the inlet gas to a middle inlet of the column gives a number of problems. In particular, this has a significant impact on the dimensions of the column. By far the largest portion of the feed gas is methane, which in the column travels upwards as a vapour. When a major portion of the supply gas is fed to one inlet, the column, above that inlet, will need to have a sufficient diameter to accommodate the upward flow of methane and heavier components in the vapour phase. To keep the methane velocities reasonable, and in known manner to prevent entrainment of the heavier component liquid droplets to be carried overhead out of the column with the methane, requires a large diameter for the column. Clearly, this diameter or cross-section will be related to the intended flow rate through the apparatus.
This requirement for a relatively large cross-section where there is a large flow of methane in the vapour phase, has often resulted in a second column being provided, as mentioned above. Thus, in view of the conventional teaching that a significant number of trays would need to be provided in any such enlarged upper section, this would often result in a top section for the column that was simply too large to be supported on top of the lower section. For this reason, such a section was often provided as a separate column.
Other hydrocarbon gas separation techniques and apparatus can be found in U.S. Pat. No. 3,702,541 (Randall et al.), U.S. Pat. No. 4,519,824 (Huebel) and U.S. Pat. No. 5,566,554 (Vijayaraghavan et al.). All of these patents disclose relatively complex techniques. What is also striking about all of these three proposals is that no portion of the inlet gas stream is taken off and fed separately to the fractionating column. Rather, it appears that the inventors, in all of these cases, have assumed that it is advantageous to achieve some separation of the inlet gas, before feeding this into the fractionation column. The assumption appears to be that if there is some initial separation, e.g. liquid/vapour separation, then this will improve the overall performance of the system. Thus, all of these proposals provide at least one separator in which vapour and gas phases are separated.
The Randall et al. patent is of interest, since the inlet gas flow is split into three separate streams, by way of liquid and vapour separators, to provide three separate inlets to the fractionation column, these being provided at various levels in the lower stripping section of the column. The column here is provided with an upper rectifying section of larger diameter. Gas taken off from the top of the column is separated into liquid and vapour fractions, and the liquid fraction is pumped back up to the top of the column, and fed into the top of the rectifying section.
Again, there is no discussion of the size problems where a substantial portion of inlet gas is fed in below the top of the fractionation column, resulting in a substantial flow upwards through the top part of the column. This problem becomes particularly acute, where a plant is designed for large flow rates. Thus, the present invention is intended to provide a plant or apparatus suitable for a flow rate as large as a thousand (1000) MMSCFD, approximately equivalent to 116500 lb moles/hr, i.e. a flow rate that is an order of magnitude or more greater than that in some of the prior art proposals discussed above. In conventional design practices, for such a large flow rate, the upper part of the fractionation column can require diameters approaching 20 feet, and it is impractical to support such large diameter sections on top of much smaller diameter lower sections.
Another common characteristic of all of these earlier proposals is that there is no detailed investigation or consideration of the behaviour at the top of the fractionation column. Conventional teaching is that a substantial disengagement section needs to be provided above the top trays, to ensure adequate and complete separation of vapour and liquid phases, so as to ensure that liquid droplets are not carried over by the vapour flow leaving the top of the fractionation column. As detailed below, on an actual plant implementation of this invention, the size of the disengagement space was reduced from 101/2 to 21/2 feet and no liquid carry over has been observed during several months of operation.